Carbon anode for aluminum reduction cell



CARBONANODE FOR ALUMINUM REDUCTION CELL Filed March 22. 1965 Sheet of 2 FIG! INVENTOR. WAYNE H. CLUKEY ROBERT L.ALBOUCO LAN V.

ATTORNEY May 6, 1969 w, CLUKEY ET AL 3,442,786

CARBON ANODE FOR ALUMINUM REDUCTION CELL Filed March 22, 1965 Sheet 2 of 2 LOW VELOCITY sPLArrE/ SPRAY 0.5

THICKNESS OF COATING 11v INCHES PERCENT ALUMINA IN CAP METAL FIG. 2

' 11/011 VELOCITY Aron/2W0 SPRAY PERCENT AL O IN CAP METAL 1 v F I G. 3

THICKNESS OF COATING IN INCHES INVENTOR.

WAYNE H. CLUKEY ROBERT L. ALBOUCQ ALAN v. 6 ACK ATTORNEY z United States Patent 3,442,786 CARBON AN ODE FOR ALUMINUM REDUCTION CELL Wayne H. Clukey, Robert L. Alboucq, and Alan V. Clack,

Spokane, Wash., assignors to Kaiser Aluminum &

Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed Mar. 22, 1965, Bar. No. 441,671 Int. Cl. B01k 3/08 U.S. Cl. 204-290 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the spraying of aluminum metal in the molten state. More specifically, this invention relates to the application of a sprayed aluminum coating on prebaked carbon anodes used in an electrolytic cell for the production of aluminum to protect the anode from air burning during operation of the electrolytic cell.

In the production of aluminum by the conventional electrochemical process, the electrolytic cell comprises in general a steel shell with a carbon bottom lining which with a layer of electrolytically produced molten aluminum which collects thereon serves as the cathode. One or more consumable carbon electrodes is disposed from the top of the cell and is immersed at its lower extremity into a layer of molten electrolyte which is disposed in the cell. In operation, the electrolyte or bath which is a mixture of alumina and cryolite, is charged to the cell, and an electric current is passed through the cell, from the anode to the cathode via the layer of molten electrolyte while oxygen collects at the anode. A crust of solidified electrolyte and alumina forms on the surface of the bath, and this is usually covered over with additional alumina.

In the conventional electrochemical process, use has been made of two types of electrorlytic cells, namely, that commonly referred to as a pre-brake cell and that commonly referred to as a Soderberg cell. With either cell, the reduction process involves precisely the same chemical reactions. The principal diiference is one of structure. In the pre-brake cell the carbon anodes are pre-baked before being installed in the cell, while in the Soderberg cell or continuous anode cell the anode is baked in situ, that is, it is baked during operation of the electrolytic cell, thereby utilizing part of the heat generated by the reduction process. The instant invention is particularly applicable to the pre-bake cell.

The oxygen deposited at the anode reacts with the hot carbon thereof to form carbon dioxide which to some extent is subsequently reduced to carbon monoxide by the hot carbon. The theroretical carbon consumption at 100% current efficiency and with the generation of pure CO is 0.333 pound carbon per pound of aluminum. The carbon consumption may be considered as an electrochemical reaction occurring at 100% current efiiciency while the cathode reaction occurs at 70-90% current efiiciency. Therefore, in order to divorce the carbon consumption from the cathode reaction the theoretical carbon consumption should be corrected for cathode current efiiciency. It should also be corrected for carbon quality which runs about 98% in the conventional prebaked anodes. Allowance is made for this loss of carbon by employing larger or taller anodes than required at the outset of operation and therefore a portion only of the anode is initially submerged in the electrolyte. As the carbon anode is consumed, the anode is lowered into the bath, generally by mechanical or automatic means, to maintain the desired anode-cathode separation in the bath. Obviously, the taller the anode being used, the fewer the number of anode changes required in replacing consumed anodes and the fewer the anodes that will have to be used in the cell.

The part of the electrode above the molten bath, being exposed to the atmosphere, will air burn if not protected in some manner. Partial protection is provided by the alumina ore cover on the electrolyte crust but as taller anodes are coming into increased usage in the industry, a better method of protecting the anode from air burn is required. Several approaches to the problem have been developed. In Europe, frequently a cast aluminum cap is placed around the exposed portion of the anode to protect it from air burn. Another method of protection used in Europe is to apply molten aluminum in a coarse heavy spray to the surface of the anode that is exposed to the atmosphere. This method of protection is mentioned in U.S. Patent 3,053,748 to P. Morel et al. assigned on its face to Pechiney. A U.S. solution to the problem has been to adhesively bind an aluminum foil sheet to the exposed portion of the anode. This method of protection is shown in U.S. Patent 3,060,115 to W. E. Haupin et al. assigned to Aluminum Company of America.

In the high load and high current density prebaked cells used in this country, the limited space between the anodes as well as the high temperature at which they operate precludes the use of cast aluminum caps or even a coarse heavy spray of molten aluminum as is used in Europe. To maintain working clearance and to provide caps that will not melt and run together, a thin non-porous coating must be applied which will resist melting early in the life of the anodes and protect the anode from air burn. The foil wrap method of Haupin is useful for this purpose. However, since the foil must be produced and then applied with an adhesive, the economics of this approach are not too satisfactory. In fact the economics are such that the reduction plants own molten metal should be used to protect the anode from air burn. By the present invention, a protective coating on an anode for an aluminum reduction cell is provided by introducing molten aluminum into a directed air stream at a temperature and under conditions to produce an atomized liquid aluminum spray, regulating the period of exposure to said directed air stream to produce an oxide content within a certain critical range in said molten aluminum and depositing said liquid spray as a coating on the anode.

It has also been found that the amount of aluminum oxide present in the aluminum coating on the anode is critical to the protective qualities of the coating. When not enough aluminum oxide is present in the aluminum coating on the anode, the coating melts because of the heat of the bath and therefore, affords little, if any, protection. When too much aluminum oxide is present in the aluminum coating on the anode the coating will crack, thereby exposing the carbon anode to the atmosphere where it will then air burn. When the right amount of aluminum oxide is present in the aluminum coating the anode, it neither cracks nor burns and, therefore, protects the anode from oxygen. It was discovered in the instant invention that the minimum amount of aluminum oxide that must be present in the coating is about 1%. When light coatings of aluminum are employed, that is coatings up to 6 of an inch thick, the aluminum oxide in the aluminum coating may be as high as When heavy coatings are placed on the anode, that is, coatings between & of an inch and V8 of an inch thick, a maximum of about 3% aluminum oxide can be in the aluminum coating. It is believed that the aluminum oxide is dispersed throughout the wvhole mass of the coating on the anode. Although the exact mechanism of how the oxide present facilitates the protection of the anode by the coating is not completely understood, it may be that the oxide particles hold the aluminum metal together as it melts from the heat of the cell and prevents the molten aluminum from falling off the anode exposing the bare carbon anode to the atmosphere.

The principal design criteria of the spray apparatus is that it discharges a directed stream of air. Many suitable spray nozzles are available commercially.

FIGURE 1 is a schematic diagarm of one such apparatus which has been successfully used in this invention. The apparatus has a hopper 11 which is filled with molten aluminum from holding furnace 12 by pouring spout 13. A molten aluminum stream pours from hopper 11 through feed channel 14 and is intercepted by a smooth flowing directed stream of air from nozzle 15 connected by air line 16 to a source 17 of pressurized air. The smooth flowing directed stream of air from nozzle 15 breaks up the aluminum stream and blows the molten aluminum against anode 18. Anode 18 is placed on turntable 19 mounted on spindle 20 for rotation so that the surface of anode 18 may be uniformly coated.

The amount of aluminum oxide in the aluminum deposited on the anode depends upon the metal temperature, the rate of molten metal flow, the air pressure and how far the molten aluminum is carried in the air stream from the nozzle before it strikes and deposits on the anode. Thus, control of composition is effected by controlling the manner of depositing the aluminum on the anode. This type of apparatus produces a rather coarse low oxide spray which splatters on the anode surface. It is also possible to use a higher velocity spray system so long as it also discharges a directed stream of air. Apparatus such as that known in the trade as the Metco Flame Spray system is useful for this purpose as at some point between a rather coarse low oxide spray and an ultrafine highly oxidized spray it is necessary to envelop the pattern of molten metal in a reducing gas flame to prevent excessive oxidation and to keep the aluminum molten. Thus, with the flame spray apparatus the protective coating on the anode is produced by introducing the molten aluminum into an air stream at a temperature and under conditions to produce a disintegrated or atomized liquid aluminum spray, passing the spray through a reducing flame and regulating the period of exposure to the non-turbulent air blast to produce the desired oxide content in the molten aluminum and depositing said liquid spray as a coating on the anode to be protected.

The invention may be further illustrated by the following example where carbon anodes 15 /2 inches wide by 20 /2 inches long by 12% inches high were coated in accordance with this invention. For a better understanding of these examples reference is made to the accompanying figures where FIG. 2 is a graph of the relationship of spray thickness and percentage of aluminum oxide in the coating where the low velocity splatter spray is used and FIG. 3 shows the relationship of the thickness of the coating to the percentage of aluminum oxide present in the metal when the high velocity disintegrater or atomizer spray is used. Using the low velocity splatter spray system, the anode was sprayed with a coating of metal 0.120 inch thick. As shown in FIG. 2, a coating of this thickness contains 3 /2% aluminum oxide. The anode was operated in a 60,000 ampere aluminum reduction cell having substantially no alumina blanket or other means to afford protection against air burning. After the cell had been in operation for only a short period of time, cracks developed in the coating and air burning of the carbon anode was noted. In the next test, the amount of aluminum oxide present in the capped metal was reduced to 3% and the anode placed in the cell and operated as before. Here no cracking occurred and the aluminum sprayed coating remained imperivous throughout the operation of the cell with the anode. In the next example, still using the low velocity splatter spray, a coating having 1% aluminum oxide was used. The results were comparable to those from the 3% aluminum oxide coating. In another test with the low velocity splatter spray, a coating containing 0.8% aluminum oxide was applied to an anode which was then operated as above in a 60,000 ampere aluminum reduction cell. In a short period of time, melting of the coating was noted and bare patches of the carbon anode began to show through. As the cell was continued to be operated, excessive air burn of the anode resulted.

In the next series of example, a high velocity disintegrating or atomizing spray wherein the molten metal was enveloped in a reducing flame to control the oxidation was used. The spray nozzle used was a modification of that known in the trade as the Metco Flame Spray system. With this system, a coating containing 3 /3% aluminum oxide was applied to the same type anode as used in the preceeding tests. When this cell was operated, the coating adhered satisfactorily to the anode and remained impervious throughout the test. No excessive air burn of the anode was noticed. In the next example using the high velocity disintegrating or atomizing spray, the thickness of the coating was increased to 0.0158 inch of thickness resulting in 10% aluminum oxide in the coating. When an anode with this coating was operated in the 60,000 ampere cell as before, a slight amount of cracking of the coating was noticed. When the amount of oxide present in this coating was increased in the next example to 11%, more extensive cracking of the coating when the anode was used in the operating cell was noticed and excessive air burn resulted. In the next example, a molten aluminum coating 0.0044 inch thick was applied to an anode using the high velocity disintegrating or atomizing spray. By means of the reducing gas flame, the oxide quantity was controlled to 5% aluminum oxide in the coating. This anode was used as before in a 60,000 ampere aluminum reduction cell. No melting or cracking of the coating was noted and no excessive air burn of the anode resulted therefrom.

The table below represents potline scale tests coparing a potline of standard 15 x 20 /2 x 12% inch anodes operated with no air burn protection other than an alumina ore cover with a standard anode coated in accordance with this invention using a low velocity splatter spray with the oxide content being 3%. Also listed in the table are comparisons of the standard anode with taller anodes, i.e., an anode 15%" tall and an anode 18" tall protected in accordance with this invention by use of the low velocity splatter spray with a coating containing 3% aluminum oxide. In this table, the carbon efficiency value is defined as the corrected theoretical carbon consumption as discussed previously divided by the actual consumption reported in percents. The tabulated results show clearly that as a result of coating the anode in accordance with this invention, the carbon efliciency rises greatly and is 13- 14% higher in all cases. It is also noted from the table that aluminum production increases greatly. Moreover the number of pot shifts per anode also increases significantly. These improvements in cell operation result in an immense economics savings since production costs are greatly reduced not only because of the decreased consumption of carbon but also because of the decrease in manpower costs to replace anodes as their life is greatly increased by practice of the invention.

It will be understood that various changes, modifications and alterations may be made in the instant invention without departing from the spirit and scope thereof and therefor the invention is not to be limited except as by the appended claims.

TABLE Standard Coated Coated Coated Anode dimensions, 15%x20% 15V x20% 15%x20% 15% x20% inches x12% x12% x15% x18 Line amperage 59, 000 60, 500 62,000 62,000 Current efliciency,

percent 81. 5 82 83.0 83. 0 Net lbs. Al/pot day 853 880 913 913 Pot shifts/anode 20 23 30 38 Lbs. carbon/lb. Al 0.590 0. 490 0. 485 0. 490 Corrected theo- .t 0. 413 0. 411 0. 406 0. 406 Carbon Cons., carbon efliciency, percent- 70. 0 83.8 83. 6 82. 8

What is claimed is:

l. A carbon anode for an aluminum reduction cell having an aluminum coating thereon having an oxide content of from 1 to 10% by weight in said aluminum.

2. The carbon anode of claim 1 on which the coating is u to about 4 of an inch thick.

3. The carbon anode of claim 1 on which the coating UNITED STATES PATENTS 3,016,447 1/1962 Gage et a1. 3,017,119 1/1962 Gibson. 3,053,748 9/1962 Morel. 3,073,720 1/1963 Mets 117-105 ALFRED A. LEAVI'IT, Primary Examiner. J. H. NEWSOME, Assistant Examiner.

US. Cl. X.R. 

